1. Field of the Invention
The present invention relates to refractive surgical systems using low-power, infrared and ultraviolet lasers in a predetermined scanning patterns in procedures of photorefractive keratectomy (PRK), laser assisted in situ keratomileusis (LASIK) and laser sclera expansion (LASE), a new procedure for presbyopia correction.
2. Prior Art
Refractive surgeries (or corneal reshaping) including a procedure called photorefractive keratectomy (PRK) and a more recent procedure called laser assisted in situ keratomileusis, or laser intrastroma keratomileusis (LASIK) have been performed mainly by lasers in the ultraviolet (UV) wavelength of (193-213) nm. The commercial UV refractive lasers include ArF excimer laser (at 193 nm) and other non-excimer solid-state lasers such as the one patented by the present inventor in U.S. Pat. No. 5,144,630. Laser corneal reshaping has been conducted by two major beam deliver techniques: the broad beam systems based on patents of L'Esperance et. al. In U.S. Pat. Nos. 4,773,441, 5,019,074, 5,108,338 and 5,163,934; and the scanning small-beam systems based on patent of the present inventor, U.S. Pat. Nos. 5,520,679 and 5,144,630.
Precise, stable corneal reshaping require lasers with strong tissue absorption (or minimum penetration depth) such that thermal damage zone is minimum (less than few microns). Furthermore accuracy of the procedure of vision correction depends on the amount of tissue removed in each laser pulse, in the order of about 0.5 microns. Therefore lasers at UV wavelength (193-213 nm) and at the mid-infrared (2.8-3.2) microns are two attractive ranges which match the absorption peak of protein and water, respectively. UV lasers however have some concerns regarding to the long-term mutagenic effects of the corneal tissue which is less concern in infrared (IR) lasers, noting that the DNA absorption peaks at a UV wavelength about 260 nm. Moreover, UV laser systems suffer problems such as optical damage of the coated mirrors, unstable and short lifetime of the lasing gases and high cost toxic gas of fluorine (for excimer laser). Low beam delivery efficiency and complexity of beam uniformity are other drawbacks of UV refractive lasers.
Refractive surgery using a scanning device and lasers in the mid-infrared (mid-IR) wavelength was first proposed by the present inventor in U.S. Pat. Nos. 5,144,630 and 5,520,679 and later proposed by Telfair et. al., in U.S. Pat. No. 5,782,822, where the generation of mid-IR wavelength of (2.5-3.2) microns were disclosed by various methods including: the Er:YAG laser (at 2.94 microns), the Raman-shifted solid state lasers (at 2.7-3.2 microns) and the optical parametric oscillation (OPO) lasers (at 2.7-3.2 microns). These mid-IR wavelength lasers are proposed as candidates for corneal reshaping due to their strong water absorption.
The present inventor had spent more than five years without success in an attempt to develop mid-IR laser systems based on the above-described prior arts. At the present time, there is no any commercial or clinically practical mid-IR refractive laser system been developed based upon the prior arts because of the inherent problems and difficulties to be discussed as follows.
Er:YAG lasers at long-pulse (or the fundamental mode without Q-switched) were proposed and described for refractive surgery by Seller and Wollensak in "Fundamental mode photoablation of the cornea for myopic correction", Laser and Light in Ophthalmology, pp. 199-203 (1993), and Cozen et al, in PCT Application No. 93/14817. No clinically acceptable results were obtained based on these prior arts because of the following drawbacks: (a) the basic Er:YAG laser having a pulse duration of about 200 microseconds, which was too long to minimize the thermal damage down to that of the UV laser range of few microns, thermal damage zone of about 20-100 microns were reported in long-pulse Er:YAG; b) only low laser repetition rate of about (5-10) Hz are available which limits the procedure speed and to a non-scanning mode; (c) uniform, flat-top beam profiles are not available and only the fundamental Gaussian-type profiles were used in vision corrections limited to myopia only;, other uses of hyperopic and astigmatism corrections can not be performed by the fixed Gaussian beam profile; (d) a 90% Gaussian or better beam profiles without any hot spot are critically required to achieve the expected corrections whose profiles are not predictable, (e) system was operated by a broad beam mode of spot (5-6 mm diameter) which required high laser energy of at least 20 mJ on the corneal surface.
Q-switched Er:YAG lasers at short pulse duration as proposed by Lin and Telfair et al., has not been developed with laser parameters meeting the clinically desired criteria of short pulse width (less than 80 nanoseconds for minimum thermal damage), high repetition rate (at least 40 Hz for reasonable surgery speed) and reliable system components. Development of Q-switched Er:YAG system was inherently limited by factors of optical damage of the Q-switching components, coating problems due to strong water absorption, and the low repetition rate Oess than 25 Hz) due to the cooling problems of the laser rod. To overcome all these inherent drawbacks in an Er:YAG system will not be cost effective and a high maintenance efforts will be required when it is used for refractive surgery. The prior art of Telfari et al proposed a short pulse, less than 50 nanoseconds, Er:YAG system has never been achieved so far in any commercially available system. The reported Er:YAG pulse duration was limited to about 200 nanoseconds.
Another alternative proposed by Lin and Telfail et al., the OPO-laser also had technical difficulties in making a clinically practical system. At this time only low repetition rate OPO-laser (lower than 30 Hz) at low energy (less than 5 mi per pulse) was tested due to the problems of: low conversion efficiency from near-IR to mid-IR wavelength, crystal and optics coating damage at high power and unstable output IR energy due to cooling problems. Therefore a practical OPO-laser system for refractive surgery will be either difficult to make or high cost at high maintenance efforts.
In addition to the above-described OPO-laser, the present inventor also had attempt at no success to develop a Raman-shifted laser due to difficulties of: unstable IR output due to Raman gas flow, optical damage of the coated windows and the inherent back scattering of the Raman signals. Again, a Raman-laser for refractive surgery will be of high cost and difficult to maintain. In addition the system can not be compact in size due to the one-meter long Raman cell.
Corneal reshaping may also be performed by laser thermal coagulation currently conducted by a Ho:YAG laser (at about 2 microns in wavelength) which however, was limited to low-diopter hyperopic corrections. A new procedure for presbyopic correction has recently proposed by implant or diamond knife incision by Schaker in U.S. Pat. Nos. 5,529,076 and 5,722,952. These prior arts, however, have drawbacks of being complexity and time consuming surgery and having risk of side effects. Using lasers for presbyopic corrections or improvements have not been previously proposed.
The above described prior arts which are not clinically practical for refractive uses because of the inherent technical problems or being not cost-effective. In light of this, it is an object of the present invention to provide new laser sources for refractive surgeries and offer the advantages of: compact, low-cost, easy to maintain, operated at mid-IR wavelengths and matching most or all of the clinically desired laser parameters (short pulse, high repetition rate and high water/tissue absorption). These new lasers proposed in this invention will match the two major water absorption peaks at about (2.7-32.) microns and (5.6-6.2) microns.
It is yet another object of the present invention to provide refractive laser systems which offer smooth ablated corneal surface by appropriate beam overlapping and scanning patterns. To further reduce the thermal effects and even corneal hydration, specially designed scanning patterns will be proposed in this invention. Furthermore the exact IR laser beam profiles are not critical in a scanning mode which provides smoother corneal surface by the randomized averaging procedure than the non-scanning lasers which require a 90% Gaussian or better profile without hot spot.
It is yet another object of the present invention to provide refractive laser systems which offer variable beam spot size of about (0.1-2.5) mm for flexible and accurate correction profiles. In the proposed designs, one single system at variable laser beam spots will provide multiple-application including PRK, LASIK and laser sclera expansion (LASE) as introduced earlier. Variable spot size (VSS) controller using electronic shutter or motorized pin-hole will be used in the presently proposed novel devices. It should be noted that the new procedure of LASE has not been reported so far in any UV or IR laser systems because it requires a scanning flexibility of the beam direction, position and the controlled beam spot size and energy for accurate ablation depth on the sclera tissue. We note that the LASE procedure is a method for the treatment of presbyopic patient by corneal sclera expansion caused by removal of a portion of the sclera tissue. After the LASE treatment the patient's accommodation will increase to see both near and far. One of the critical issues of LASE is the corneal bleeding during the laser "cutting". Therefore it is yet another objective of this invention is to combine a coagulation laser with the ablation laser to prevent or minimize the bleeding in the LASE procedure.
According to the discussion in Jacques, S. L., Laser-tissue Interactions; Photochemical, Photothermal and Photomechanical, Lasers in General Surgery, 72t3), 531-558 (1992), we note that photoablation is associated with the UV excimer laser corneal ablation, whereas photomechanical ablation is responsible for the mid-IR laser interaction with the corneal tissue. In the present invention, however, we shall just use the name of "ablation" for the process of photomechanical ablation in describing the mid-IR radiation interaction with the corneal, where corneal tissue fragments was by the mid-IR lasers by its strong absorption in water. We further note that a laser pulse duration of less than 50 nanoseconds for corneal thermal damage claimed by the prior art of Telfair et al may be true for the PRK or LASIK procedures, in which few microns of tissue are removed in each mid-IR pulse. However, an Er:YAG laser Q-switched to a short pulse of 50 nanoseconds has not The condition of short pulse less than 50 nanoseconds not required in the present invention for PRK or LASIK procedures. We propose in this invention that new mid-IR lasers with pulse duration of about (10-100) nanoseconds instead. We also propose that when a laser beam is tightly focused, for example less than 0.15 mm, a long pulse laser may still ablate corneal tissue efficiently at a minimum thermal damage. For the proposed LASE procedure, a much longer laser pulse (in about 10 microseconds or longer) may be used for efficient sclera tissue ablation, as far as the beam spot is less than 150 microns on the corneal surface.
It is yet another object of this invention is to provide novel devices with variable beam spot sizes and multi-stage scanning such that the refractive surgical procedures can be performed within (10-30) seconds without losing the profile accuracy.
It is yet another object of the present invention to provide refractive laser systems which offer a "gas blower" on the corneal surface during the surgery to maintain a controlled level of corneal hydration, which is rather sensitive and critical to the laser ablation rate which is mainly caused by the water absorption in the tissue. The hydration condition of the corneal surface is much more important in IR lasers than in U-V lasers for the process of corneal reshaping.